Controlling vibrational excitations in $\rm \bf C_{60}$ by laser pulse durations

Two similar off-resonant ultrafast laser experiments [1-3] in $\rm C_{60}$ have reported two different vibrational modes that dominate the relaxation process: one predicts the ag modes while the other the hg modes. A systematic simulation presented here reveals that this experimental discrepancy results from the laser pulse duration. The numerical results show that since each mode $\nu$ has a distinct optimal duration $\tau_o^\nu$, the ag modes are strongly suppressed for durations longer than 40 fs, while the hg modes start to grow. For the off-resonant and low-intensity excitations, the period $\Omega^o_\nu$ of the dominant mode and $\tau_o^\nu$ satisfy the relation $\Omega_\nu^o/{\tau}_o^\nu \approx 3.4$. By carefully scanning the laser frequencies and pulse durations, a comprehensive excitation diagram is constructed, which can be used to guide experiments to selectively excite the ag and hg modes in cm by an ultrafast laser [4,5]. Its potential impact is also discussed. \newcommand{\prl}{{\small \it Phys. Rev. Lett.} } \newcommand{\prb}{{\it Phys. Rev. B} } \newcommand{\et}{{\it et al. }} \newcommand{\ete}{{\it et al.}} [1] S. Dexheimer \ete, {\it Ultrafast Phenomena VIII}, edited by J. L. Martis \ete, {\it Springer Series in Chemical Physics} {\bf 55}, 81 (1993). [2] V. R. Bhardwaj \ete, \prl {\bf 91}, 203004 (2003). [3] H. Hohmann \ete, \prl {\bf 73}, 1919 (1994). [4] G. P. Zhang and T. F. George, \prl {\bf 93}, 147401 (2004). [5] G. P. Zhang, \prl {\bf 91}. 176801 (2003).